I don't think this is possible with WebFlow hosting... would require self-hosting.
There's a couple routes you could follow.
1) Form sends an email message to client. Client opens / read / copies content of the email to a text file (data). Client formats the text file (ie: places delimiters into data) and saves file. Client imports formatted data file into excel.
2) More difficult to do... bypass the email message. Instead... submit the form to a php script. PHP script would then save a format text file or (optional... save the data into a db / table).
3) Harder to do... bypass the email message. Submit to a php script. PHP script would generate an .XLS formatted file. Client simply opens the Excel file - in Excel. No importing.
So pretty much... option 1 is you send the data... Client does a lot of manual work to achieve the end-result.
Option 2 and 3... Client has a much better day - especially if they are doing this task on a regular basis.
Options 2 and 3 are very similar. Biggest difference is - with option 2... you create a data file in an importable format. With option 3... you actually create an excel formatted file.
for examples with option 2:
php save to excel format